I thought the answer was "ARPANET" since we had a NCP on 4th edition Unix in late 1974 or early 1975 from the University of Illinois dating from that time (the code in TUHS appears to be based on V6 + a number of patches).
Because we can't ask Greg sadly, I think the Holmgren is the last around that would know definitively and I've personally lost track of him.
That said, I don't think UofI had anything earlier than 5th edition (I fairly sure that there were very few copies of 4th edition distributed outside of the Bell: i.e. Columbia, NYU and I thought Harvard; but I don't think too many more than that). Lou Katz would be a better source than I, but I was always under the impression that the number 5th editions, the count was also a smaller 2 digit integer. 6th was where Unix began to 'spread' and by 7th, 'go viral.'
And to be honest, I personally thought that Steve and Greg did the ArpaNet NCP work on V6, but it might have been v5th I suppose. I did not know about it until the 6th edition work. But, they were fairly early. BTW: I thought the Rand PIPE code was also developed on 6th, but those also might have been 5th.
Clem